Breathe heavily (4)
I believe the answer is:
pant
'breathe heavily' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pant that I've seen before include "Breathe hard" , "Breath rapidly" , "Breath noisily from exertion" , "Long" , "Breathe noisily after running" .)